U.S. patent application Ser. No. 812,283; filed July 1, 1977, describes a process for making sintered polycrystalline CBN compacts which utilizes pyrolytic HBN (PBN) in the absence of any catalyst such as those specified in U.S. Pat. No. 2,947,617. A compact is a mass of abrasive particles bonded together either: (1) in a self-bonded (see U.S. Pat. Nos. 3,852,078 and 3,876,751) relationship; (2) by means of a bonding medium (see U.S. Pat. Nos. 3,136,615; 3,233,988; 3,743,489; 3,767,371 and 3,918,931); or (3) by means of some combination of (1) and (2). U.S. Pat. No. 3,918,219 teaches the catalytic conversion of HBN to CBN in contact with a carbide mass to form a composite body. CBN compacts are comprised of a plurality of CBN crystals suitably bonded together to form a large, integral, tough, coherent, high-strength mass. Compacts are used in such applications as machining, dressing and drilling (see U.S. Pat. Nos. 3,136,615 and 3,233,988).
A method for the conversion of HBN to CBN in the absence of catalyst is described in U.S. Pat. No. 3,212,852 (100 kilobars and 3600.degree. K.)--see also: Wakatsuki, et al., "Synthesis of Polycrystalline Cubic BN (VI)", and Ichinose, et al., "Synthesis of Polycrystalline Cubic BN (V)", both in Proceedings of the Fourth International Conference of High Pressure, Kyoto, Japan (1974), pp. 436-445; U.S. Pat. No. 4,016,244; Wakatsuki et al., Japanese Patent No. Sho 49-27518; Wakatsuki et al., Japanese Patent No. Sho 49-30357; Wakatsuki et al., Japanese Patent No. Sho 49-22925; Wakatsuki et al., U.S. Pat. No. 3,852,078; Wakatsuki et al., "Synthesis of Polycrystalline Cubic Boron Nitride", Mat. Res. Bull., 7, 999-1004 (1972); and Sirota, N. British Patent No. 1,317,716.
British Patent No. 1,513,990 discusses the production of a cubic boron nitride compact prepared by high pressure-high temperature processing of mixtures of hexagonal boron nitride and boron powder.
An article by Corrigan and Bundy ("Direct Transition Among the Allotropic Forms of Boron Nitride at High Pressures and Temperatures", The Journal of Chemical Physics, Vol. 63, No. 9 (Nov. 1, 1975) p. 3812) discusses the effect of impurities (e.g., oxygen) in the high pressure-high temperature process for converting hexagonal boron nitride to cubic boron nitride at page 3814.
The heating of boron nitride to temperatures ranging from 1200.degree.-2000.degree. C. is reported to evolve nitrogen gas and leave a coating of boron in Dreger, L. H., et al., "Sublimation and Decomposition Studies on Boron Nitride and Aluminum Nitride", J. Phys. Chem., 66 (1962) p. 1556.
Vacuum firing of isotropic hexagonal boron nitride to remove boron oxide prepatory to metallizing is mentioned in U.S. Pat. No. 3,926,571; col. 3.
Preliminary drying of HBN is disclosed in U.S. Pat. No. 4,150,098, see col. 3.
A method for producing aggregate abrasive grains for cutting tools, (through sintering a mixture of abrasive powders, metal alloy powders, and an adhesion-active agent to produce a cake which is subsequently crushed) is disclosed in U.S. Pat. No. 4,024,675.
Two forms of hexagonal boron nitride have been identified, turbostratic and ideal hexagonal or graphitic (GBN). The turbostratic structure is characteristic of pyrolytic boron nitride and is a continuous structure characterized by 2-dimensional layers of hexagonal rings stacked at irregular intervals and randomly oriented.
PBN is a low pressure form of HBN made typically by chemical vapor deposition of BCl.sub.3 +NH.sub.3 vapors on a graphite substrate. As deposited, it has a high purity of 99.99+%, a density between about 2.0 and 2.18 g/cm.sup.3 (compared to 2.28 for GBN), and a preferred orientation of the layer planes between 50.degree. and 100.degree. in the [001] direction (c-axis).
The structure of PBN, as with analogous pyrolytic carbon in the carbon system, is not well understood. Various models have been proposed to explain the structure of PBN and pyrolytic carbons. According to one of the more popular models, termed turbostratic state, the B and N atoms form more or less parallel stacks of fused hexagonal BN layers, but with stacking being random in translation parallel to the layers and random in rotation about the normal to the layers. Other models emphasize imperfections and distortion within the layers. The increased interlayer spacing in the pyrolytic materials (3.42 A for PBN compared to 3.33 A for GBN) is attributed primarily to the disorder in the stacking direction resulting in attenuation of the weak van der Waals attraction between the layers. The structure in a mass of PBN is continuous in any given direction, as opposed to being separated by crystal boundaries.
The "as deposited" type of PBN will be referred to hereinafter as unrecrystallized PBN (U-PBN).
Another known type of PBN is recrystallized PBN (R-PBN). It is formed by compression annealing of PBN and has a theoretical density of 2.28 g/cm.sup.3, a highly crystalline structure with an interlayer spacing of 3.33 A, a purity of 99.99+%, and a preferred layer plane orientation of about 2.degree. or less in the [001] direction (c-axis). R-PBN is further described in U.S. Pat. No. 3,578,403.
Also, the aforementioned U.S. Pat. No. 3,212,852, col. 10, ll. 19-24, discloses the use of PBN as a starting material in direct conversion processes practiced at pressures above 100 kbars.
The layers of hexagonal rings in the graphitic form (GBN) are highly oriented giving a material which is soft, flaky and transparent. Further details on the two forms of HBN may be found in Thomas, J. et al., "Turbostratic Boron Nitride, Thermal Transformation to Ordered-layer-lattice Boron Nitride", J.A.C.S., Vol. 84, No. 24 (Jan. 25, 1963) p. 4619; and Economy, J., and Anderson, R., "Boron Nitride Fibers", J. Polymer Science: Part C, No. 19, (1967) p. 283.